Screw-and-plate osteosynthesis systems must allow immobilizing of one or more bone fragments in reference to others. It is known to use spherical-head screws cooperating with a spherical aperture housed in a plate and bringing the plate into compression over the bone until the friction of the plate on the bone stabilizes the assembly. These assemblies allow choice of the angle of implantation of screws during the operation and causing a return movement and a compression of a detached bone fragment. Certain of these systems allow, due to the oblong shape of the aperture made in the plate, compression of one bone fragment on another. The shortcoming of these systems is their low resistance to compression stresses exerted parallel to the plane of the plate.
Use is also known of a second generation of screw-and-plate systems called monoaxial-locking and polyaxial-locking systems in which the strength of the assembly no longer depends on compression of the plate on the bone but on a fixation of the screw in the plate. These systems allow achieving assembly away from the bone with, for the more elaborate ones, the possibility of choosing the angle of implantation of screws during the operation while achieving strength sufficient for postoperative stresses.
Commonly owned U.S. Patent Publication No. 2005/0143742 (“the '742 publication”), the disclosure of which is hereby incorporated by reference herein, teaches an improvement to the above-noted and already useful devices. More particularly, the '742 publication teaches a device which is used for solidly connecting a part such as a plate to an underlying support (e.g., a bone) using at least one fixing element such as screw. According to the invention of the '742 publication, the fixing element takes the form of a threaded rod or screw which passes through a hole housing a ring belonging to the part, such that it is screwed into the support material. The ring is preferably a constriction ring including a non-circular outer profile which co-operates with the non-circular inner profile of the hole formed in the part. Upon rotation of the ring, such is wedged in place and constricted and thereby blocking the threaded rod or screw against movement with respect to the plate.
Even in view of the innovative and highly useful device taught in the '742 publication, which is itself an improvement upon well-known and useful technology, there still exists room for improved functionality and design in fixation of bone plates. For example, there exists a need for improved bending strength between the threaded rod or screw and the plate, among other improvements.